Robustness of superconductivity to structural disorder in Sr$_{0.3}$(NH$_2$)$_{y}$(NH$_3$)$_{1-y}$Fe$_2$Se$_2$

The superconducting properties of a recently discovered high-Tc superconductor, Sr/ammonia-intercalated FeSe, have been measured using pulsed magnetic fields down to 4.2 K and muon spin spectroscopy down to 1.5 K. This compound exhibits intrinsic disorder resulting from random stacking of the FeSe layers along the c axis that is not present in other intercalates of the same family. This arises because the coordination requirements of the intercalated Sr and ammonia moieties imply that the interlayer stacking (along c) involves a translation of either a/2 or b/2 that locally breaks tetragonal symmetry. The result of this stacking arrangement is that the Fe ions in this compound describe a body-centered-tetragonal lattice in contrast to the primitive arrangement of Fe ions described in all other Fe-based superconductors. In pulsed magnetic fields, the upper critical field Hc2 was found to increase on cooling with an upward curvature that is commonly seen in type-II superconductors of a multiband nature. Fitting the data to a two-band model and extrapolation to absolute zero gave a maximum upper critical field μ0Hc2(0) of 33(2)T. A clear superconducting transition with a diamagnetic shift was also observed in transverse-field muon measurements at Tc≈36.3(2)K. These results demonstrate that robust superconductivity in these intercalated FeSe systems does not rely on perfect structural coherence along the c axis.